Olefins such as ethylene, propylene or the like are produced by thermally cracking material gases of hydrocarbons (naphtha, natural gas, ethane, etc.). The thermal cracking reaction is conducted by introducing the hydrocarbon material gas and steam into a cracking coil disposed within a heating furnace supplied with heat from outside, and heating the mixture to a reaction temperature range while the mixture flows through the coil at a high velocity.
Typically, the cracking coil comprises a plurality of (straight) tubes which are connected into a zigzag assembly by bends.
To conduct the thermal cracking reaction efficiently, it is important to heat the fluid flowing inside the coil at a high velocity to the reaction temperature range radially inward to the central portion of the tube channel within a short period of time and to avoid heating at a high temperature to the greatest possible extent. If the gas is heated at a high temperature over a prolonged period of time, lighter fractions of hydrocarbons (methane, free carbons, etc.) will be produced in excessive amounts or the product of cracking will undergo, for example, a polycondensation reaction to reduce the yield of the desired product. Promoted coking (deposition of free carbon on the tube inner wall) will also result to lower the coefficient of heat transfer, giving rise to a need to perform decoking frequently.
Accordingly it is practice to provide fins on the tube inner surface of the cracking coil as elements for stirring the fluid within the tubes. The fluid flowing at a high velocity produces turbulence by being stirred by the fins, and can be heated to a higher temperature rapidly. As a result, the reaction is completed within a shortened period of time, while production of lighter fractions due to excessive cracking is avoided. Furthermore, an improvement in the coefficient of heat transfer of the tubes makes it possible to lower the temperature of the tubes, producing an effect to improve the serviceable life of the tubes.
FIGS. 12 to 14 show in development proposed examples of fins on cracking tubes (JP-A No. 1997-241781).
FIG. 12 shows fins 1 continuously extending helically at a constant angle of inclination with the tube axis.
FIG. 13 corresponds to the continuous helical fins of FIG. 12 as formed discretely. Fins 1 and nonfin portions 2 on helical loci are in a staggered arrangement wherein the fins are replaced by nonfin portions every turn of helix.
These examples have a great effect to stir the fluid within the tubes and are highly efficient in heat transfer to the fluid within the tubes, whereas the internal pressure of the fluid inside the tubes builds up owing to a great pressure loss of the fluid, entailing the drawback that the cracking operation produces ethylene, propylene or the like in a lower yield.
FIG. 14 shows fins 1 and nonfin portions 2 arranged alternately on a plurality of lines parallel to the tube axis. However, the fins positioned in parallel to the tube axis fail to produce a sufficient effect to stir the fluid inside the tubes and to achieve the desired heat transfer performance.
In view of the above problems, an object of the present invention is to suppress pressure losses to the greatest possible extent while maintaining an effect to promote heat transfer to the fluid within the tube.